The present invention relates generally to a cutter holder attached to a spindle of a machine tool for supporting a cutting tool rotated by the spindle. More particularly, the invention is concerned with such a holder for a rotary cutting tool, which causes minimum decrease in accuracy of machining by the tool due to a positioning error of the holder with respect to the machine spindle.
Various tool holders have been used for mounting drills, milling cutters, reamers, boring bars and other rotary cutting tools on spindles of machine tools such as drilling, milling and boring machines, and machining centers which are capable of automatically performing multiple kinds of cutting operations. Such a tool holder supports a tool at its one end portion, and is adapted to be removably fixed to the machine spindle at the other end portion.
However, contact or positioning surfaces of the tool holder and the machine spindle are liable to wear due to frequent mounting and dismounting of the tool holder on and from the machine spindle. Further, these tapered surfaces are subject to cutting chips, dust and dirt, and other foreign matters. As a result, the tool holder does not always make a sufficiently snug fit in the spindle bore. Thus, it has been difficult or impossible to avoid a positioning error of the tool holder due to misalignment or inclination of its centerline with respect to the centerline of the spindle. This positioning error or misalignment of the tool holder results in an increase in run-out of a rotary cutting tool at the free end of the tool holder, and consequently leads to machining errors such as an oversize of a bore diameter to be cut by the tool. That is, the positioning error of the tool holder lowers the accuracy of machining with the tool.
The above indicated positioning error of a tool holder will give rise to a serious trouble, particularly in fine-boring, reaming and other machining operations which require a relatively high accuracy.
In the light of the above problem, the inventors of the present application and their co-worker proposed, in the pending patent application Ser. No. 625,960 filed June 29, 1984 (assigned to the assignees of the present application), a tool holder for a rotary cutting tool which is capable of preventing a decrease in machining accuracy due to a positioning error of the tool holder with respect to a machine spindle. This tool holder has a holder body fixed to the machine spindle, and a rotatable shaft disposed concentrically with the holder body and having a tool mounting portion at its one end. The rotatable shaft is coupled at its other end portion to the holder body for receiving a torque from the holder body and is axially and radially displaceable relative to the holder body. Further, the tool holder has a positioning member fixedly disposed radially outwardly of the spindle and a casing disposed radially outwardly of the rotatable shaft such that the casing and the rotatable shaft are rotatable relative to each other. The casing is engageable with the positioning member for accurate positioning thereof by the positioning member, and thereby positioning the rotatable shaft, while the holder body is fixed to the spindle.
In such a tool holder, a rotary cutting tool is fixed to one end portion of the rotatable shaft. The rotatable shaft is positioned by the casing, which is accurately positioned by the positioning member disposed fixedly around the outer circumference of the free end of the machine spindle. Accordingly, a possible misalignment of the holder body with respect to the spindle will not affect a machining accuracy of the rotary cutting tool carried by the rotatable shaft. In other words, the accurate positioning of the casing permits high-precision machining, irrespective of the mounting accuracy of the holder body relative to the spindle.
However, there is a continuing requirement for further improvement in machining accuracy, particularly in reaming, fine-boring and other high-precision machining operations. Thus, there is a continuing demand for tool holders for rotary cutting tools for such machining operations, which contribute to further improvement in the machining accuracy.